1. Field of the Invention
This invention relates to a digital signal magnetic recording/reproducing apparatus for use in digital video tape recorders and so on.
2. Description of the Prior Art
In conventional digital signal magnetic recording/reproducing apparatuses, due to the difficulty of recording/reproducing direct current components, baseband modulation techniques have been used for minimizing the direct current component in the recording signals, such as NRZ codes (e.g., J. K. R. Heitmann "An Analytical Approach to the Standardization of Digital Video Tape Recorder", SMPTE J., 91, 3, Mar. 1982, or J. K. R. Heitmann "Digital Video Recording, New Result in Channel Coding and Error Protection", SMPTE J., 93: 140-144, Feb. 1984) , 8-10 block codes (e.g., J. L. E. Baldwin "Digital Television Recording with Low Tape Consumption", SMPTE J., 88: 490-492, July, 1979), Miller-square (M.sup.2) codes (e.g., L. Gallo "Signal System Design for a Digital Video Recording System", SMPTE J., 86: 749-756, Oct. 1977), ternary partial response system, etc.
However, in the digital signal magnetic recording/reproducing apparatuses utilizing such baseband modulation techniques, due to the basic system of recording the binary signals, the utilization efficiency of the recording frequency band (transmissible bit rate per unit band) is low. For instance, if the roll-off rate of a Nyquist transmission system to be 0.5, then the utilization efficiency of the frequency band is at most 1.33 bits/sec./Hz. This incurs an increased tape consumption, providing an impediment to long time recording. Further, in order to increase the recording rate, a method of expanding the recording frequency band, a method of increasing the number of recording channels, or a method of increasing the relative velocity have been considered. However, if the recording frequency band is expanded, the S/N ratio is significantly deteriorated. Accordingly, the recording rate cannot be increased very much. When the number of recording channels is increased, the track width is narrowed and the S/N ratio is deteriorated. If the relative velocity is increased, the amount of tape consumption naturally increases.
To compensate for the deterioration of the S/N ratio, the error rate may be improved by using error control coding. However, the bit rate of the data which can be actually recorded is lost by such coding (Ref. L. M. H. E. Dreissen et al., "An Experimental Digital Video Recording System", IEEE Conf. Jun. 1986, or C. Yamamitsu et al., "An Experimental Digital VTR Capable of 12-hour Recording", IEEE Trans. on CE, CE-33, No. 3, pp. 240-248, 1987).
On the other hand, there has been proposed a coded modulation scheme effected by combining a multi-value modulation scheme, which has a high utilization efficiency of the frequency band, with error control coding for application to digital data transmission. This, however, does not give consideration on the special conditions such as the effect of non-linear distortion or the effect of saturation characteristics in the case of recording/reproducing on a magnetic recording medium (cf. G. Ungerboeck "Channel Coding with Multilevel/Phase Signals", IEEE Trans. on IT, IT-28, No. 1, pp. 55-67, 1982).
We have already proposed, as disclosed in the U.S. patent application Ser. No. 07/302,325, a digital signal magnetic recording/reproducing apparatus using an amplitude phase shift keying technique which can provide a high frequency efficiency and recording rate. When intending to record/reproduce digital signals being subjected to the amplitude phase shift keying, however, in order to obtain an error rate at a practical level (about 10.sup.-4), it is necessary to increase the S/N ratio, resulting in it being impossible to expand the frequency band of the recording signal. That is, only a part of the frequency band of a signal capable of being recorded on a magnetic recording medium can be utilized. As a result, only a slightly higher recording density than would be attained with a baseband modulation technique can be provided.